The escalating demands for high density and performance associated with non-volatile memory devices require small design features, high reliability and increased manufacturing throughput. The reduction of design features, however, challenges the limitations of conventional methodology.
One type of conventional electrically erasable programmable read only memory (EEPROM) device includes a silicon substrate with an oxide-nitride-oxide (ONO) stack formed on the substrate. A silicon control gate is formed over the ONO stack. This type of memory device is often referred to as a SONOS (silicon-oxide-nitride-oxide-silicon) type memory device. In a SONOS device, the nitride layer acts as the charge storage layer.
The SONOS type memory device may be programmed by a hot electron injection process that injects electrons into the nitride layer where they become trapped. The nitride layer may then store this negative charge.
The SONOS type memory device is typically erased using a hot hole injection (HHI) process. In the HHI process, voltages may be applied to the memory device to cause hot holes to move into the nitride layer, effectively erasing the negative charge on the nitride layer. Fowler-Nordheim (FN) erase mechanisms that are used in conventional floating gate memory devices (e.g., memory devices with polycrystalline floating gate electrodes) are not typically used in SONOS type memory devices due to back gate injection problems. That is, during the erase process, as electrons tunnel from the nitride layer into to the substrate, electrons also move from the control gate through the top oxide layer to the nitride layer. These electrons from the control gate effectively counter the loss of electrons tunneling through the bottom oxide layer. Therefore, FN erasing is not typically performed for a SONOS type memory device.